Optimized spatial filter for defect detection

ABSTRACT

An apparatus for detecting the defects composed of non-linear components of a subject pattern, includes a laser source for illuminating the subject pattern by a laser beam, a Fourier-transform lens for projecting an information light from the pattern through a spatial filter onto a screen. The filter is placed on the Fourier-transform plane of the Fourier-transformed information light for intercepting the coherent light having information of the linear components, the filter having arm sections extending correspondingly to the linear components of said normal pattern, the outer periphery of said arm sections including curved sections protruding toward the intersecting point of the arms.

BACKGROUND OF THE INVENTION

This invention relates to an apparatus for detecting defects of apattern with directional characters such as IC photo masks, and morespecifically to an apparatus for detecting defects employing a spatialfilter for filtering optically coherent light.

A detector for detecting defects of patterns with a directionalcharacter such as IC photo masks as well as positional informationthereof by applying incoherent light and coherent light to the patternon the same optical axis has already been proposed by the inventorshereof (U.S. Pat. No. 3,972,616 filed on June 27, 1975; patented on Aug.3, 1976). The device disclosed in the said U.S. Pat. No. 3,972,616 is anavailable means for detecting the position of a defect on an IC mask.Because of the shape of the spatial filter as a component of the deviceof the invention, there still remain such problems as the fact that itis difficult to increase the S/N ratio of the brightness (S) of a defectimage to the noise (N) or the light intensities of pattern images in theoutput plane other than the defects to an adequate degree as well as ashortcoming that the noise level may vary substantially with the changeof the subject pattern in size.

SUMMARY OF THE INVENTION

An object of this invention is to provide an apparatus for detecting thedefects of the pattern with directional characters which may presenthigher performance or higher S/N ratio as well as low fluctuations innoise level irrespective of any change of the subject pattern in size bysolving the aforementioned problems.

In an aspect of the present invention an apparatus for detecting thedefects of a pattern including a normal pattern composed of linearcomponents and the defects composed of non-linear components comprises acoherent light source radiating coherent light, means for leading thecoherent light radiated from said light source to the pattern toilluminate the pattern, a Fourier-transform lens forFourier-transforming information light from said pattern obtained byilluminating the pattern with the coherent light, a spatial filterplaced on the Fourier-transform plane of the Fourier-transformedinformation light and intercepting the coherent light having informationof the linear components of the pattern, the filter having arm sectionsextending correspondingly to the linear components of said normalpattern, the outer periphery of said arm sections including curvedsections protruding toward the intersecting point of the arms, and meansfor projecting a coherent image transmitted through said spatial filter.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention can be more fully understood from the following detaileddescription when taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a diagram giving an outline of the apparatus for detecting thedefects of the pattern according to the invention;

FIGS. 2A and 2B are views for illustrating the shapes of conventionalspatial filters;

FIGS. 3A, 3B, 3C and 3D are views for illustrating the shapes of spatialfilters available for the device of the invention;

FIG. 4 is a diagram for illustrating the spatial filter of FIG. 3A indetail;

FIGS. 5A and 5B are respectively views for illustrating the shapes ofthe normal pattern and defect pattern used as standards for measurementof S/N ratio;

FIGS. 6A, 6B and 6C are graphs showing the measurement results of therelation between the subject pattern sizes and the output intensitiesfor the cases of using the spatial filters as shown in FIGS. 2A, 2B and3A, respectively;

FIG. 7 is a view for showing another shape of spatial filter availablefor the device of the present invention;

FIG. 8 shows another embodiment of the device of the present inventionemploying the spatial filters as shown in FIGS. 3A through 3D and FIG.7.

FIGS. 9A and 9B respectively show further embodiments of the device ofthe present invention employing the spatial filters as shown in FIGS. 3Athrough 3D and FIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

There will now be described an apparatus for detecting the defects of apattern according to an embodiment of this invention with reference toFIGS. 1, 3A and 4.

Referring to FIG. 1, numeral 1 indicates a coherent light sourcecomposed of, e.g., a He-Ne laser source, from which coherent light isradiated, converted into a parallel light 3 with a predetermineddiameter by a collimeter 4 comprising a pair of optical lenses 5 and 6,and then rendered incident upon a half mirror 8. The coherent light 3incident upon the half mirror 8 is transmitted through the mirror 8 andapplied to a subject pattern 7 composed of a photo mask for use on an ICarranged across the optical axis. Also used as the subject pattern 7 maybe striped shadow mask for a cathode-ray tube.

Meanwhile, incoherent light radiated from an incoherent light source 2composed of, e.g., an incandescent lamp, after selection of a light witha specific wavelength range by a blue or green color filter 12, isconverted into a parallel light 13 with about the same diameter as theaforementioned paralleled coherent light 3, by a collimator 9 comprisinga pair of optical lenses 10 and 11 and then rendered incident upon thehalf mirror 8. The incident light 13 is reflected by the mirror 8 tohave an optical axis identical with that of the coherent light 3 asshown in the figure and then rendered incident upon the subject pattern7.

The photo mask or shadow mask to be used as the subject pattern 7 iscomposed of a pattern including transparent and opaque sections.

The light transmitted through the subject pattern by the incidence ofthe coherent light 3 and incoherent light 13 on the information area ofthe subject pattern is condensed by an optical lens 15 arranged in afiltering optical system. The lens 15, placed at a distance of 2F (F isa Focal length of the lens 15) from the position of the subject pattern,may function as a Fourier transform lens for the coherent informationlight component of the light transmitted through the subject pattern 7.Accordingly, the coherent information light is Fourier-transformed bythe lens 15 and takes a spectral distribution corresponding to theconstruction of the subject pattern 7 on the Fourier transform surface.That is, as described above, when the subject pattern 7 is composed onlyof longitudinal and transverse straight lines, the said informationlight has a spectral distribution with directivity in the horizontal andvertical directions. Meanwhile, if the subject pattern 7 has anydefects, the spectra of the defects will generally have no specificdirectivity.

Applying the principle that the Fourier spectrum of a normal pattern isdirectional, while that of a defect is non-directional, the directionalpattern information from the subject pattern 7 is intercepted by adirectional filter 16 placed on the condensing side (spectrum side) ofthe lens 15 and the defect information alone is imaged on a screen 17through an imaging lens (served by the lens 15 in this embodiment),thereby detected as a defect.

As for the incoherent information light component of the transmittedlight from the subject pattern, it has not such a directionaldistribution as that of the coherent information light in the vicinityof the focal plane of the lens 15, so that it cannot provide a definitefiltering effect at the image plane, thereby projecting the whole imageof the subject pattern 7 on the screen 17.

Consequently, the existence of defects as well as the positions thereof,may be detected on the screen 17 by the positional information on thepattern obtained from the incoherent light and the defect information onthe pattern from the coherent light.

When the inspection of defects in the irradiated area of the subjectpattern 7 is completed in such a manner as mentioned above, the pattern7 is shifted by a pattern feeding mechanism (mask driver) 14 andinspected for another area thereof.

The spatial filter 16 for use on the aforementioned device is of such aconstruction as shown in FIG. 3A.

There will now be described the spatial filter with reference to FIGS.2A and 2B illustrating conventional spacial filters and to FIGS. 6A, 6Band 6C indicating the characteristics of these three filters.

A filter 20a as shown in FIG. 2A is of a cross-shape with a coherentlight intercepting area extending horizontally and vertically, and afilter 20b of FIG. 2B has a shape as stated in the aforementioned U.S.Pat. No. 3,972,616. For ease of description, S/N ratio for each case ofusing the filters of FIGS. 2A and 2B as well as that of FIG. 3A will beexplained.

S/N ratio as mentioned herein is intended to S/N ratio on the basis ofoptical signals where S is a brightness of an image of defect, while Nis a brightness appearing on the output surface as a result of the lightmainly from the several corners of the subject pattern 7 that is notfiltered completely. That is, although the subject pattern may becomposed of linear elements alone, its corner sections (as indicated bynumerals 51, 52, 53 and 54 in FIG. 5A) are roundish microscopically andsuch roundness is of the same geometric nature as a defect, so that saidfilter may not intercept the rays from said corners, allowing them toappear on the output plane as noises. Though, strictly speaking, abrightness N may of course include any other brightness such as that ofmisguided rays, it exists in a substantially negligible amount.

In measuring S/N ratio, a square pattern with a luminous intensity of 1as shown in FIG. 5A was taken as a standard normal pattern, while acircular pattern with a luminous intensity of 1 as shown in FIG. 5B wastaken as a standard defect pattern. A brightness S was to be given atthe minimum value of output for each defect pattern size, while abrightness N was to be given at the maximum value of output for eachnormal pattern size. The results are as shown in FIGS. 6A, 6B and 6C.FIG. 6A illustrates the relation between pattern size l and outputluminous intensity in the case of using the filter as shown in FIG. 2A,while FIGS. 6B and 6C illustrate the relation between pattern size andoutput luminous intensity in the cases of using the filters as shown inFIGS. 2B and 3A respectively. That is, as may be seen from thesemeasurement results, the output luminous intensity may fluctuate withthe pattern size l at a variable fluctuating ratio. As for S/N ratio,S/N=2.03 for FIG. 6A (i.e., where the filter of FIG. 2A is used),S/N=2.89 for FIG. 6B (i.e., where the filter of FIG. 2B is used), andS/N=6.05 for FIG. 6C (i.e., where the filter of FIG. 3A is used).

That is, it is revealed that the S/N ratio with the filter of FIG. 3A isincreased by about three times and twice as compared with those with thefilters of FIG. 2A and FIG. 2B respectively. Further, the fluctuatingratio of the noise light (leakage light from a normal pattern imagethrough the filter) according to pattern size Nmax/ Nmin is about 2.3for the filter of FIG. 2A and about 2.1 for the filter of FIG. 2B, whilethat for the filter of FIG. 3A is about 1.2, indicating that the filterof FIG. 3A has a far lower fluctuating ratio of noise luminous intensityas compared with two other filters.

Thus, when either of the filters as shown in FIGS. 2A and 2B is used,the light from the corner sections of IC mask pattern break through thefilter to appear as noises on the output surface, thereby reducing S/Nratio at time of defect detection of the output surface. These noisesmay vary with the size (construction) of the IC mask pattern. In thisway, when the filters as shown in FIGS. 2A and 2B are used, S/N ratio isrelatively low and the noises vary substantially with the pattern size,so that the threshold level required for judging defects cannot helpbeing set at an extremely high point in detecting defects, e.g.,electrically. That is, in case of the filter as shown in FIG. 2A, thefluctuating ratio of noise light is about 2.3 as mentioned above, sothat the threshold level is required to be set at a point about 2.3times higher than the observed noise light level in order to cut anoptional noise. If the threshold level is set at such a point, however,S/N ratio of the filter will be about 2.03 as mentioned above, so it ishighly probable that the signal level to indicate a defect will alsofall under the threshold level, thereby giving much difficulty in defectdetection.

As for the filter as shown in FIG. 2B, the fluctuating ratio of noiselight is about 2.1. Accordingly, the threshold level is required to beset at a point about 2.1 times higher than the observed noise lightlevel. In this case, however, S/N ratio is about 2.89 as mentionedabove. Therefore, defect detection may be performed more easily than inthe case of using the filter of FIG. 2A, though it may not be deemed tobe satisfactory, considering the effects of, e.g., fluctuations oflasers, drift of the detector, and shading.

Further, in performing visual inspection, ease of defect determinationmay be reduced for similar reasons.

Meanwhile, when the filter as shown in FIG. 3A is used, S/N ratio isrelatively high as may be seen from FIG. 6C and the fluctuating ratio ofnoise according to the pattern size is as low as 1.2. Accordingly, withsuch a filter, S/N ratio of the filter will be as high as 6.05 if thethreshold level is set at a point about 1.2 times higher than theobserved noise light level, allowing satisfactory defect detection to beperformed.

There will now be described in detail the shape of the filter 16 asshown in FIG. 3A with reference to FIG. 4.

The center C of a coherent light intercepting area 16a of the filter 16is on the same optical axis with the center of the effective pupil ofthe lens 15, and arm sections 16b extend in the horizontal and verticaldirections from the center. The outer periphery of these arm sections16b includes hyperbolic sections 16c protruding toward the center C.That is, where X- and Y-axes are taken in the horizontal and verticaldirections respectively as shown in FIG. 4, the coherent lightintercepting area 16a of the filter will comply with

    (|X|-A)(|Y|-A) ≦B.sup.2 (1)

here A and B are positive constants, and | | denotes the absolute value.

In other words, the light intercepting area is a portion enclosed withthe hyperbolas as drawn in the four quadrants with lines X=±A and Y=+Aas asymptotes. Computer simulation has revealed that when the effectivepupil on the filter plane is a square with a side length of L asindicated by the dot lines in FIG. 3A, a relatively high S/N may beobtained where A×B≃1/64 L in accordance with formula (1), and themaximum S/N may be obtained where A=7/64 L and B=9/64 L.

The computer simulation has also indicated that when the effective pupilon the filter plane is a circle with a diameter of L as indicated by thedot-and-dash lines in FIG. 3A, a relatively high S/N may be obtainedwhere A=B≃40/4096 L² to 49/4096 L², and the maximum S/N may be obtainedwhere A=7/64 L and B=7/64 L in accordance with formula (1).

There will now be described further embodiments of the spatial filtersavailable for the device of the present invention with reference toFIGS. 3B, 3C, 3D and 7.

The outer periphery 16c of the intercepting area 16a of the filter 16shown in FIG. 3B includes a part of a circle with a radius of R, where Ris given as an optional value within a range of (0<)R<L/2 and aprescribed width 2A of the arm 16b is determined.

If the radius R is smaller than that shown in FIG. 3B, the filter couldbe formed with the shape as shown in FIG. 3C.

Further, the outer periphery 16c of the intercepting area 16a of thefilter 16 of FIG. 3D includes a part of an ellipse with the major axisof r₁ and the minor axis of r₂.

FIG. 7 shows another embodiment of the spatial filter available for thedevice of the invention that may be used when the normal pattern(subject pattern without any flaws) includes oblique elements (at anangle of 45° to the horizontal axis, for example) as well as thevertical and horizontal components. In the filter 16 as shown in FIG. 7,there are provided arm sections 16e extending at an angle of 45° to thehorizontal (or vertical) axis besides the arm sections 16b extending inthe horizontal and vertical directions in the filters as shown in FIG.3A, 3B and 3C in the intercepting area 16a. Further, the outer peripheryside of the intersecting portion between each two adjacent arms 16b and16e is composed of a curve protruding in the direction toward the centerof the filter.

As may be seen from FIG. 7, if the normal pattern includes anycomponents extending in any predetermined directions other than thedirection of the horizontal axis, there may be formed arm sectionsextending at an angle corresponding to such components in the coherentlight intercepting area.

FIG. 8 shows another embodiment of the apparatus of the presentinvention employing the spatial filters as shown in FIGS. 3A, 3B, 3C and7. In FIG. 8, any parts identical with the corresponding ones as shownin FIG. 1 are indicated by the same numerals.

The apparatus as shown in FIG. 8 may be used suitably when the subjectpattern 7 is not liable to transmit light. That is, if the subjectpattern is something like a wafer, an element in the IC manufacturingprocess, the defect detecting for the wafer may be conducted byemploying the reflected light from the wafer. The principle and methodof flaw detection are similar to the case as described with reference toFIG. 1.

FIGS. 9A and 9B show further embodiments of the apparatus of the presentinvention. In the same manner as the case of FIG. 8, any parts identicalwith the corresponding ones as shown in FIG. 1 are indicated by the samenumerals, and the description thereof will be omitted.

Referring now to FIG. 9A, numeral 91 indicates a beam splitter with aplurality of holes at the peripheral portions. The beam splitter isarranged between the coherent light source 1 and the collimator 4, androtated by a driving source 92 such as a motor at a speed as low as 10c/s. Consequently, the coherent light is rendered upon the collimator 4intermittently, providing on-and-off indication of defect image on thescreen 17. This system is specially effective in performing visualdefect detection, facilitating operator's judgment.

Referring now to FIG. 9B, numeral 95 indicates a liquid crystal plate.The liquid crystal plate 95 is controlled by a liquid crystal drivingsource 96. That is, the amplitude of the coherent light radiated fromthe coherent light source 1 may be modulated intermittently with theliquid crystal plate 95 by driving the liquid crystal plate 95intermittently with the liquid crystal driving source 96. Consequently,in the same manner as the case described with reference to FIG. 9A, adefect image is subjected to on-and-off indication on the screen 17.When using the liquid crystal plate, the light radiated from thecoherent light source 1 may be scattered by the liquid crystal plate, sothat it is advisable to interpose a pair of lenses 97 and 98 and a plate99 with a pinhole placed substantially on the focus plane of the lens 97between the liquid crystal plate 95 and the collimator 4 to obtain abetter on-off characteristics as shown in FIG. 9B.

Further, the intermittent interception of coherent light as illustratedby FIGS. 9A and 9B may be also performed by means of the coherent lightsource 1 itself. In this case the coherent light source 1 should bedriven intermittently.

Although the filters as shown in FIGS. 3A, 3B, 3C and 7 may be made ofconventional materials such as color glass and multi-layer dielectricfilms, they should preferably be formed by using gelatin films for easeof preparations or the like.

Furthermore, in the aforementioned embodiments, coherent and incoherentlight are used for detecting both the defect itself the and positionthereof and for detecting position alone respectively, though theincoherent light need not be used in detecting the existence of defectsalone.

As described above in detail, in the defect detector of this invention,the outer periphery of the intersecting portion between each twoadjacent arms of the coherent light intercepting area of the spatialfilter is of a curve-shape protruding in the direction toward the centerof the filter, so that it may not only intercept securely luminousinformation of patterns with directional characters but enlargeefficiently the area through which the luminous information indicatingdefects is transmitted. Consequently, according to the device of thisinvention, S/N ratio may be increased, while the fluctuating ratio ofnoise luminous intensity may be reduced.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:
 1. An apparatus for detecting defects in a patternhaving linear straight line features and nonlinear defectscomprising:(a) a coherent light source for radiating a coherent light;(b) a collimator for collimating the coherent light radiated from saidcoherent light source into a light beam with a predetermined diameterand leading it to said pattern; (c) a transform lens for transformingthe intensity distribution of transmitted or reflected light from saidpattern into a Fourier-transformed pattern; and (d) a spatial filterplaced on the Fourier-transform plane of said transform lens andpreventing transmission of the coherent light having information of saidlinear straight line features of said pattern, said filter having alight intercepting area defined by a plurality of arm sections extendingin the horizontal and vertical directions from a common point ofintersection correspondingly to the linear straight line features ofsaid subject pattern, a portion lying between two adjacent arms being ofa hyperbola-shape protruding toward the center of said filter whereinthe coherent light intercepting area of said spatial filter is definedaccording to

    (|X|-A)(|Y|-A)≦B.sup.2

where X and Y indicate the X-axis and Y-axis of said filterrespectively, said X- and Y-axes corresponding to the respective axes ofthe arms of said filter extending in the horizontal and verticaldirections, X=Y=0 is indicative of the center of said filter, and both Aand B are positive constants, wherein said constants A and B are givenas values complying with

    A×B≃1/64 L

where L is the side length of an effective pupil or the diameter of aneffective pupil of said spatial filter.
 2. An apparatus according toclaim 1 wherein said constants A and B are given substantially at 7/64 Land at 9/64 L to 7/64 L, respectively.
 3. An apparatus according toclaim 1 wherein said spatial filter comprises a gelatin filter.
 4. Anapparatus according to claim 1 which further comprises an incoherentlight source radiating incoherent light and means for leading theincoherent light from the light source to said pattern in compliancewith the optical axis of said coherent light.
 5. An apparatus accordingto claim 4 wherein said coherent light source is a Helium-Neon laser andsaid incoherent light source is an incandescent lamp.
 6. An apparatusaccording to claim 5 which further comprises a color filter locatedacross the optical path of the incoherent light radiated from saidincoherent light source.
 7. An apparatus according to claim 6 whereinsaid color filter is a blue or green gelatin filter.
 8. An apparatusaccording to claim 1 which further comprises control means forintermittently blocking the coherent light radiated from said coherentlight source.
 9. An apparatus according to claim 8 wherein said controlmeans includes a beam splitter with a plurality of holes spacedsubsequently to said coherent light source and driving means forrotating said splitter.
 10. An apparatus according to claim 8 whereinsaid control means includes a liquid crystal plate placed subsequentlyto said coherent light source, a lens system for collimating thecoherent light transmitted through said liquid crystal plate, and aplate with a pinhole arranged in said lens system.